I think the Dolphins were a little better than 27th in the NFL against the pass, but let's also remember that the Dolphins were very stout against the run so teams took advantage of the weaker part of Miami's defense.

ESPN wrote:

Miami Dolphins: The Dolphins were 27th against the pass last season and needed to make changes. Miami was aggressive in free agency and the draft this year, making a big signing at corner in former Pro Bowler Brent Grimes and spending second- and third-round picks on corners Jamar Taylor and Will Davis, respectively. Grimes has easily been the most impressive corner in organized team activities and minicamp. He is 100 percent again following last year’s season-ending Achilles injury. Taylor and Davis both have a lot to learn, but the Dolphins hope at least one rookie is up to speed and ready to contribute by the start of the regular season. Veteran cornerback Richard Marshall is starting with Grimes. The safety positions are the same with Reshad Jones and Chris Clemons. Jones, in particular, has the potential to be one of the better safeties in the AFC.